The risks to babies born prematurely are great and the economic costs to society continue to increase. At present, infants less than 22 weeks gestation are not predicted to survive, and at 28 weeks, only 77% survive. The total yearly cost for preterm babies in the U.S. is in excess of five billion dollars. In order for preterm babies to have a better chance of survival, premature labor must be detected prior to any dilation of the cervix, so that effective drug therapy can be instituted.
In more than 50% of cases of threatened premature labor, the patient's contractions are those of false labor and will stop spontaneously. However, when the patient calls her physician, she will be advised to go to the hospital. If, when the patient arrives at the hospital, her cervix is not significantly dilated, she will be observed for a period of time. If the contractions stop, the patient will be sent home. If, however, they continue, the cervix can dilate further and either make it more difficult to stop the labor, or make the birth of the baby inevitable. The drugs used to stop contractions of premature labor pose a significant risk, both to the mother and baby. Before a physician will start administering these drugs, he must be absolutely sure that the patient is in true labor.
Every muscle produces an electric signal when it contracts. The electric signal from the heart can be measured as an electrocardiogram (EKG), and from the uterus as an electrohysterogram (EHG). This EHG will vary during pregnancy and labor.
In my U.S. Pat. No. 5,301,680, issued Apr. 12, 1994, entitled "Apparatus and Method for the Diagnosis of Labor", there are described systems and methods that determine a periodicity of an electrical signal in an abdominal region. A uterine contraction vector having a magnitude and propagation direction is then determined and is employed to distinguish true labor from false labor, and to detect the onset of true labor. Methods and apparatus are also disclosed for determining the extent of cervical dilation.
It is known to those skilled in the art that, as labor progresses, there is a shift toward higher frequencies in the frequency spectrum of the electrical activity of the uterus.
By example, Marque and Duchene ("Human Abdominal EHG Processing for Uterine Contraction Monitoring", in Applied Biosensors, D. L. Wise Ed., Butterworth Publisher, Boston, 187-226 (1989)), have disclosed a technique for the detection of premature labor by calculating a criterion. The criterion is based on the energy in a total band (0.2-3.0 Hz), a low frequency band (0.2-0.45 Hz), a high frequency band (0.8-3.0 Hz), and the length of a recorded EHG burst. These authors calculate and graph this criterion during contractions, and show that if it remains negative it is an "effective" contraction, and if it becomes positive it is an "ineffective" contraction.
Reference can also be had to U.S. Pat. Nos. 5,446,953, 5,623,939 and 5,776,073 wherein Garfield describes the use of action potential measuring electrodes to acquire electromyographic signals. Frequency components of the electromyographic signals are analyzed to characterize uterine activity.